The present invention relates to a protective box for electronic circuits hardened with respect to X-rays. It more especially applies to protecting high performance electronic circuits used in the aeronautical and space fields against X-rays.
High-performance electronic circuits, both from the processing speed and the capacity standpoints, are very sensitive to the effects of X-rays. These effects can even lead to the destruction of the active components of the electronic circuits involving latch up phenomena.
Apart from the X-radiation dose or quantity received, a particularly important parameter is that of the time during which said quantity is supplied. The dose associated with its application time is called the dose rate. The behavior or resistance of the active components of the electronic circuits with respect to said parameters (dose, dose rate) and the energy spectrum of said radiation is essentially linked with the production technology thereof. In most cases, it is necessary to reduce the dose levels and rates in order to permit the electronic circuits to retain their functional integrity.
One of the most widely used methods for reducing doses and dose rates received by electronic circuits consists of enclosing them in an envelope made from a pure metal with a high atomic number. The metal and thickness of the metal sheet are chosen and adapted as a function of the energy of the X-radiation in question and the desired filtering rate. This metal sheet effectively protects against high X-ray doses and dose rates. As a function of the circuits and/or technology of the electronic components, the need for protection can be felt as from 1 to 10 Krad and 10.sup.5 to 10.sup.7 rad.s.
Generally, the metal sheet covers a metal structure, particularly of light alloy, enclosing the electronic circuits, said structure providing the necessary mechanical strength and protection. The metal sheet is mechanically fixed over the entire outer surface of the metal structure.
Unfortunately the realization of the most interesting metals for this type of protection is difficult and costly. Moreover, the requirements with respect to said protection materials and the guarantee that they will not deteriorate under various ionizing, mechanical and climatic surrounding conditions means that the weight breakdown of the electronic circuits is highly increased, compared with circuits which are not protected against X-rays.
In most cases, the metal sheet for protecting against X-rays cannot be engaged directly over the entire outer surfaces of the metal structure of the encapsulating box due to the often complex profile thereof. This profile complexity is often imposed by heat dissipation constraints.
Therefore the volume defined by the metal protection sheet is greater than the volume of the mechanical structure to be protected. This leads to an increase in the weight and overall dimensions of the mechanical structure, which is further increased by the mechanical devices required for maintaining the metal sheet in place on the mechanical structure (spacers, angle brackets, screws, bolts, etc.). In addition, these maintenance devices must be made from the same metal as the metal protection sheet, so as not to create "holes" in the protection against X-rays.
Furthermore, as the mechanical structure is made from a metal or alloy, this further increases the total weight of the box for encapsulating the electronic circuits.
It is clear that these disadvantages as regards the overall dimensions and weight of the encapsulating boxes are particularly prejudicial with respect to the use of electronic circuits on-board aircraft.
Another method consists of directly depositing the X-ray protection metal on the mechanical structure to be protected either by impregnating the latter in a liquid bath, or by electrolysis. However, these deposition processes are not possible for all the metals usable for providing protection against X-rays. Moreover, in this case it is also necessary to examine the corrosion compatibilities of the metals present.
In addition, the thicknesses which can be deposited for the metals lending themselves to this procedure are limited, otherwise the deposit adhesion quality may be prejudiced. Moreover, the obtaining of a homogeneous deposit makes it necessary to proceed in successive stages with further machining between the deposits so that, in certain cases, the dimensions of the encapsulating box are respected in the final stage. Thus, these deposition methods are limited and lead to a high cost of the X-ray protection encapsulating boxes.
Another approach consists of envisaging a specific protection for each on-board electronic component, which constitutes a different and more advance solution compared with those referred to hereinbefore. This specific protection described in FR-A-2 547 113, filed on 3 June 1983, consists of using several stacked layers of different materials having different atomic numbers (Z).
Materials with a high atomic number are dielectric ceramics, such as barium or neodymium titanate, titanium oxide or a complex lead-based ceramic. Materials having a low atomic number are carbon, aluminium, silicon, alumina and silica.
As a function of the applications and number of components involved, the number of individual protections can be more disadvantageous from the weight standpoint than an overall protection of all the electronic components. Moreover, the technology for producing the different materials forming the piles or stacks is based on processes used for the production of capacitors and in particular fritting processes. In particular, the process described does not make it possible to obtain an X-ray protection material with a complex shape.
Within the framework of protecting persons working in the presence of X-rays, the materials mainly comprise a charge such as lead, dispersed in an organic binder. Such protection materials are in particular described in FR-A-2 190 717, filed in the name of the Giken Company, FR-A-2 482 761, filed in the name of A. Maurin and US-A-3 622 432 of H. K. Porter Company.
These lead-based materials can only be used as X-ray protection materials for radiation with a low flow rate associated with relatively long dose distribution times.
Another known electronic circuit encapsulating box is described in FR-A-2 490 917, filed on 2 September 1980. This box is made from a moulded plastic material, such as an epoxy resin, in which the electronic circuits are embedded. This box is extremely thin and does not make it possible to effectively mechanically protect the electronic circuits. Moreover, there is no X-ray protection.
The present invention relates to a box for protecting electronic circuits hardened with respect to X-rays and making it possible to obviate the various disadvantages referred to hereinbefore. Compared with the use of a heavy metal sheet covering a metal structure, this protective box in particular leads to an important weight and overall dimension gain, whilst effectively protecting against radiation with a high dose rate and in particular exceeding those referred to hereinbefore.
Moreover, this protective box causes no major manufacturing problem and can be manufactured in a much shorter time than that necessary for manufacturing prior art encapsulating boxes.
Moreover, compared with FR-A-2 547 113, the invention makes it possible to bring about a development of the X-ray protection levels without any detrimental affect on the definition of the electronic circuits contained in the box.